Well, I did it! I mentioned Saturday that Week 4 was making me really nervous, as it includes a marked increase in jogging (after the 5 minute warmup walk, it's 3 minute jog, 90 second walk, 5 minute jog, 2 1/2 minute walk, 3 minute jog, 90 second walk, 5 minute jog.)
I'd been having some lightheadedness during some of my runs over the last couple of weeks. I talked to my dad (who is a runner), and he suggested I make sure I'm pushing all the air out when I exhale, so that I can breathe plenty of air in when inhaling. I really focused on that this time. I didn't get that icky lightheaded feeling (despite the extra jogging time), and I felt better at the end of my last 5 minute jogging segment than I had at the end of the first! It may have also helped that I ate half a banana and a spoonful of peanut butter before leaving this morning.
And the last thing that helped? I slowed down my pace even more so that my jogs were reeeeeeally slow. I can add speed later when my legs and lungs are more used to this.
I think I'm turning into a runner.